In order to clarify the microvascular architecture and ultrastructural features of the capillary vessels related to transendothelial transport of metabolites, scanning electron microscopy of tissues digested by HCl-collagenase and of vascular corrosion casts as well as thin-section, tracer, and freeze-fracture replications were employed to study the maturation zone of rat-incisor enamel organ. The enamel-organ maturation zone was shown to have a well-developed, dense capillary plexus. The capillary vessels were distributed along furrows formed by the enamel-organ papillary ridges. In central regions they formed a regular, blindlike network; in the peripheral regions, however, they formed an irregular, circular network. Everywhere except in the nuclear and perinuclear regions, the very thin capillary-vessel endothelial walls were pierced with numerous fenestrations. Such fenestrations were evident in endothelial walls facing the ameloblast-layer site. In tracer experiments, intravenously injected horseradish peroxidase passed through the fenestrations in the endothelial walls to diffuse throughout the enamel-organ extracellular spaces. It did not, however, pass through intercellular spaces or transendothelial channels. The dense, regular distribution of highly fenestrated capillaries in the enamel organ is thought to make possible the rapid transcapillary exchange of various metabolites between the vascular system and the ameloblast and papillary layers that is necessary for enamel maturation.